A live varicella vaccine will be given to children with leukemia. The immune response clinical reactivity and propensity of the vaccine virus to spread to contacts of vaccines will be evaluated. Responses will be correlated with the immune status, stage of disease and stage of chemotherapy of the vaccine recipient. The objective will be to determine whether children with leukemia can be protected against varicella by immunization without producing significant morbidity with the vaccine. An attempt will be made to determine the host factors which are associated with optimal immune response and minimal morbidity. The goal is to protect children with leukemia against varicella by immunization with live vaccine.